The present invention relates in general to the control of glass fiber forming bushings and, more particularly, to an improved method and apparatus for controlling one or more segments of a multiple segment bushing to extend the range of control utilizing a given auxiliary power level by controlling both the heating and cooling of one or more segments of the bushing.
Glass fibers are commonly produced by drawing multiple streams of molten glass through nozzles or holes located in a heated container known in the industry as a bushing. The bushing is electrically resistance heated by passing high currents through it. Since the temperature of the bushing is one important factor in determining the characteristics of the glass fibers which are produced using the bushing, a variety of temperature control arrangements have been devised. All known fiber glass bushing control circuits are based on either current diversion around all or one or more segments of a multiple segment bushing or current injection into one or more segments of a multiple segment bushing.
Temperature control using current diversion around a bushing or one or more segments of a multiple segment bushing is disclosed, for example, in U.S. Pat. No. 4,594,087. In the disclosed arrangement, a controller diverts current around a bushing or segments of a bushing using variable impedance circuits. The diverted current reduces the temperature of the bushing or segment of the bushing from which current was diverted. An improved bushing controller utilizing an auxiliary transformer in each of the variable impedance circuits of the '087 controller to increase the current capacity of the controller is disclosed in U.S. Pat. No. 4,931,075.
Temperature control using current injection into a bushing is disclosed, for example, in U.S. Pat. No. 4,780,120. In the disclosed arrangement, current is injected into all but one segment of a multiple segment bushing and adjusts the supply of electrical energy to the entire bushing to control the temperature of each segment of the bushing.
Many of the known prior art arrangements are able to control fiber glass bushings to produce high quality glass fibers. Unfortunately, all known prior art arrangements require partial activation, preferably approximately 50%, during initial start-up and balancing of a fiber glass bushing which complicates these procedures.
There is an ongoing need for improvement in controlling fiber glass bushings to improve their operation and efficiency. Preferably, an improved bushing controller would not interfere with initial start-up and balancing operations, would provide increased range of bushing control without increasing the size of an auxiliary bushing power supply and would improve the coefficient of variation of glass fibers produced by controlled fiber glass bushings.